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# Vue.js style guide

## Linting

We default to [eslint-vue-plugin](https://github.com/vuejs/eslint-plugin-vue), with the `plugin:vue/recommended`.
Please check this [rules](https://github.com/vuejs/eslint-plugin-vue#bulb-rules) for more documentation.

## Basic Rules

1. The service has its own file
1. The store has its own file
1. Use a function in the bundle file to instantiate the Vue component:

   ```javascript
   // bad
   class {
     init() {
       new Component({})
     }
   }

   // good
   document.addEventListener('DOMContentLoaded', () => new Vue({
     el: '#element',
     components: {
       componentName
     },
     render: createElement => createElement('component-name'),
   }));
   ```

1. Do not use a singleton for the service or the store

   ```javascript
   // bad
   class Store {
     constructor() {
       if (!this.prototype.singleton) {
         // do something
       }
     }
   }

   // good
   class Store {
     constructor() {
       // do something
     }
   }
   ```

1. Use `.vue` for Vue templates. Do not use `%template` in HAML.

1. Explicitly define data being passed into the Vue app

    ```javascript
    // bad
    return new Vue({
      el: '#element',
      components: {
        componentName
      },
      provide: {
        ...someDataset
      },
      props: {
        ...anotherDataset
      },
      render: createElement => createElement('component-name'),
    }));

    // good
    const { foobar, barfoo } = someDataset;
    const { foo, bar } = anotherDataset;

    return new Vue({
      el: '#element',
      components: {
        componentName
      },
      provide: {
        foobar,
        barfoo
      },
      props: {
        foo,
        bar
      },
      render: createElement => createElement('component-name'),
    }));
    ```

    We discourage the use of the spread operator in this specific case in
    order to keep our codebase explicit, discoverable, and searchable.
    This applies in any place where we would benefit from the above, such as
    when [initializing Vuex state](../vuex.md#why-not-just-spread-the-initial-state).
    The pattern above also enables us to easily parse non scalar values during
    instantiation.

    ```javascript
    return new Vue({
      el: '#element',
      components: {
        componentName
      },
      props: {
        foo,
        bar: parseBoolean(bar)
      },
      render: createElement => createElement('component-name'),
    }));
    ```

## Naming

1. **Extensions**: Use `.vue` extension for Vue components. Do not use `.js` as file extension
([#34371](https://gitlab.com/gitlab-org/gitlab-foss/-/issues/34371)).
1. **Reference Naming**: Use PascalCase for their instances:

   ```javascript
   // bad
   import cardBoard from 'cardBoard.vue'

   components: {
     cardBoard,
   };

   // good
   import CardBoard from 'cardBoard.vue'

   components: {
     CardBoard,
   };
   ```

1. **Props Naming:**  Avoid using DOM component prop names.
1. **Props Naming:** Use kebab-case instead of camelCase to provide props in templates.

   ```html
   // bad
   <component class="btn">

   // good
   <component css-class="btn">

   // bad
   <component myProp="prop" />

   // good
   <component my-prop="prop" />
   ```

## Alignment

1. Follow these alignment styles for the template method:

   1. With more than one attribute, all attributes should be on a new line:

      ```html
      // bad
      <component v-if="bar"
          param="baz" />

      <button class="btn">Click me</button>

      // good
      <component
        v-if="bar"
        param="baz"
      />

      <button class="btn">
        Click me
      </button>
      ```

   1. The tag can be inline if there is only one attribute:

      ```html
      // good
        <component bar="bar" />

      // good
        <component
          bar="bar"
          />

      // bad
       <component
          bar="bar" />
      ```

## Quotes

1. Always use double quotes `"` inside templates and single quotes `'` for all other JS.

   ```javascript
   // bad
   template: `
     <button :class='style'>Button</button>
   `

   // good
   template: `
     <button :class="style">Button</button>
   `
   ```

## Props

1. Props should be declared as an object

   ```javascript
   // bad
   props: ['foo']

   // good
   props: {
     foo: {
       type: String,
       required: false,
       default: 'bar'
     }
   }
   ```

1. Required key should always be provided when declaring a prop

   ```javascript
   // bad
   props: {
     foo: {
       type: String,
     }
   }

   // good
   props: {
     foo: {
       type: String,
       required: false,
       default: 'bar'
     }
   }
   ```

1. Default key should be provided if the prop is not required.
   There are some scenarios where we need to check for the existence of the property.
   On those a default key should not be provided.

   ```javascript
   // good
   props: {
     foo: {
       type: String,
       required: false,
     }
   }

   // good
   props: {
     foo: {
       type: String,
       required: false,
       default: 'bar'
     }
   }

   // good
   props: {
     foo: {
       type: String,
       required: true
     }
   }
   ```

## Data

1. `data` method should always be a function

   ```javascript
   // bad
   data: {
     foo: 'foo'
   }

   // good
   data() {
     return {
       foo: 'foo'
     };
   }
   ```

## Directives

1. Shorthand `@` is preferable over `v-on`

   ```html
   // bad
   <component v-on:click="eventHandler"/>

   // good
   <component @click="eventHandler"/>
   ```

1. Shorthand `:` is preferable over `v-bind`

   ```html
   // bad
   <component v-bind:class="btn"/>

   // good
   <component :class="btn"/>
   ```

1. Shorthand `#` is preferable over `v-slot`

   ```html
   // bad
   <template v-slot:header></template>

   // good
   <template #header></template>
   ```

## Closing tags

1. Prefer self-closing component tags

   ```html
   // bad
   <component></component>

   // good
   <component />
   ```

## Component usage within templates

1. Prefer a component's kebab-cased name over other styles when using it in a template

   ```html
   // bad
   <MyComponent />

   // good
   <my-component />
   ```

## Ordering

1. Tag order in `.vue` file

   ```html
   <script>
     // ...
   </script>

   <template>
     // ...
   </template>

   // We don't use scoped styles but there are few instances of this
   <style>
     // ...
   </style>
   ```

1. Properties in a Vue Component:
   Check [order of properties in components rule](https://github.com/vuejs/eslint-plugin-vue/blob/master/docs/rules/order-in-components.md).

## `:key`

When using `v-for` you need to provide a *unique* `:key` attribute for each item.

1. If the elements of the array being iterated have an unique `id` it is advised to use it:

   ```html
   <div
     v-for="item in items"
     :key="item.id"
   >
     <!-- content -->
   </div>
   ```

1. When the elements being iterated don't have a unique ID, you can use the array index as the `:key` attribute

   ```html
   <div
     v-for="(item, index) in items"
     :key="index"
   >
     <!-- content -->
   </div>
   ```

1. When using `v-for` with `template` and there is more than one child element, the `:key` values
must be unique. It's advised to use `kebab-case` namespaces.

   ```html
   <template v-for="(item, index) in items">
     <span :key="`span-${index}`"></span>
     <button :key="`button-${index}`"></button>
   </template>
   ```

1. When dealing with nested `v-for` use the same guidelines as above.

   ```html
   <div
     v-for="item in items"
     :key="item.id"
   >
     <span
       v-for="element in array"
       :key="element.id"
     >
       <!-- content -->
     </span>
   </div>
   ```

Useful links:

1. [`key`](https://vuejs.org/v2/guide/list.html#key)
1. [Vue Style Guide: Keyed v-for](https://vuejs.org/v2/style-guide/#Keyed-v-for-essential)

## Vue testing

Over time, a number of programming patterns and style preferences have emerged in our efforts to
effectively test Vue components. The following guide describes some of these.
**These are not strict guidelines**, but rather a collection of suggestions and good practices that
aim to provide insight into how we write Vue tests at GitLab.

### Mounting a component

Typically, when testing a Vue component, the component should be "re-mounted" in every test block.

To achieve this:

1. Create a mutable `wrapper` variable inside the top-level `describe` block.
1. Mount the component using [`mount`](https://vue-test-utils.vuejs.org/api/#mount)/
[`shallowMount`](https://vue-test-utils.vuejs.org/api/#shallowMount).
1. Reassign the resulting [`Wrapper`](https://vue-test-utils.vuejs.org/api/wrapper/#wrapper)
instance to our `wrapper` variable.

Creating a global, mutable wrapper provides a number of advantages, including the ability to:

- Define common functions for finding components/DOM elements:

  ```javascript
  import MyComponent from '~/path/to/my_component.vue';
  describe('MyComponent', () => {
    let wrapper;

    // this can now be reused across tests
    const findMyComponent = wrapper.find(MyComponent);
    // ...
  })
  ```

- Use a `beforeEach` block to mount the component (see
[the `createComponent` factory](#the-createcomponent-factory) for more information).
- Use an `afterEach` block to destroy the component, for example, `wrapper.destroy()`.

#### The `createComponent` factory

To avoid duplicating our mounting logic, it's useful to define a `createComponent` factory function
that we can reuse in each test block. This is a closure which should reassign our `wrapper` variable
to the result of [`mount`](https://vue-test-utils.vuejs.org/api/#mount) and
[`shallowMount`](https://vue-test-utils.vuejs.org/api/#shallowMount):

```javascript
import MyComponent from '~/path/to/my_component.vue';
import { shallowMount } from '@vue/test-utils';

describe('MyComponent', () => {
  // Initiate the "global" wrapper variable. This will be used throughout our test:
  let wrapper;

  // Define our `createComponent` factory:
  function createComponent() {
    // Mount component and reassign `wrapper`:
    wrapper = shallowMount(MyComponent);
  }

  it('mounts', () => {
    createComponent();

    expect(wrapper.exists()).toBe(true);
  });

  it('`isLoading` prop defaults to `false`', () => {
    createComponent();

    expect(wrapper.props('isLoading')).toBe(false);
  });
})
```

Similarly, we could further de-duplicate our test by calling `createComponent` in a `beforeEach` block:

```javascript
import MyComponent from '~/path/to/my_component.vue';
import { shallowMount } from '@vue/test-utils';

describe('MyComponent', () => {
  // Initiate the "global" wrapper variable. This will be used throughout our test
  let wrapper;

  // define our `createComponent` factory
  function createComponent() {
    // mount component and reassign `wrapper`
    wrapper = shallowMount(MyComponent);
  }

  beforeEach(() => {
    createComponent();
  });

  it('mounts', () => {
    expect(wrapper.exists()).toBe(true);
  });

  it('`isLoading` prop defaults to `false`', () => {
    expect(wrapper.props('isLoading')).toBe(false);
  });
})
```

#### `createComponent` best practices

1. Consider using a single (or a limited number of) object arguments over many arguments.
  Defining single parameters for common data like `props` is okay,
  but keep in mind our [JavaScript style guide](javascript.md#limit-number-of-parameters) and
  stay within the parameter number limit:

    ```javascript
    // bad
    function createComponent(data, props, methods, isLoading, mountFn) { }

    // good
    function createComponent({ data, props, methods, stubs, isLoading } = {}) { }

    // good
    function createComponent(props = {}, { data, methods, stubs, isLoading } = {}) { }
    ```

1. If you require both `mount` _and_ `shallowMount` within the same set of tests, it
can be useful define a `mountFn` parameter for the `createComponent` factory that accepts
the mounting function (`mount` or `shallowMount`) to be used to mount the component:

    ```javascript
    import { shallowMount } from '@vue/test-utils';

    function createComponent({ mountFn = shallowMount } = {}) { }
    ```

1. Wrap calls to `mount` and `shallowMount` in `extendedWrapper`, this exposes `wrapper.findByTestId()`:

    ```javascript
    import { shallowMount } from '@vue/test-utils';
    import { extendedWrapper } from 'helpers/vue_test_utils_helper';
    import { SomeComponent } from 'components/some_component.vue';

    let wrapper;

    const createWrapper = () => { wrapper = extendedWrapper(shallowMount(SomeComponent)); };
    const someButton = () => wrapper.findByTestId('someButtonTestId');
    ```

### Setting component state

1. Avoid using [`setProps`](https://vue-test-utils.vuejs.org/api/wrapper/#setprops) to set
component state wherever possible. Instead, set the component's
[`propsData`](https://vue-test-utils.vuejs.org/api/options.html#propsdata) when mounting the component:

    ```javascript
    // bad
    wrapper = shallowMount(MyComponent);
    wrapper.setProps({
      myProp: 'my cool prop'
    });

    // good
    wrapper = shallowMount({ propsData: { myProp: 'my cool prop' } });
    ```

    The exception here is when you wish to test component reactivity in some way.
    For example, you may want to test the output of a component when after a particular watcher has
    executed. Using `setProps` to test such behavior is okay.

### Accessing component state

1. When accessing props or attributes, prefer the `wrapper.props('myProp')` syntax over
`wrapper.props().myProp` or `wrapper.vm.myProp`:

    ```javascript
    // good
    expect(wrapper.props().myProp).toBe(true);
    expect(wrapper.attributes().myAttr).toBe(true);

    // better
    expect(wrapper.props('myProp').toBe(true);
    expect(wrapper.attributes('myAttr')).toBe(true);
    ```

1. When asserting multiple props, check the deep equality of the `props()` object with
[`toEqual`](https://jestjs.io/docs/expect#toequalvalue):

    ```javascript
    // good
    expect(wrapper.props('propA')).toBe('valueA');
    expect(wrapper.props('propB')).toBe('valueB');
    expect(wrapper.props('propC')).toBe('valueC');

    // better
    expect(wrapper.props()).toEqual({
      propA: 'valueA',
      propB: 'valueB',
      propC: 'valueC',
    });
    ```

1. If you are only interested in some of the props, you can use
[`toMatchObject`](https://jestjs.io/docs/expect#tomatchobjectobject). Prefer `toMatchObject`
over [`expect.objectContaining`](https://jestjs.io/docs/expect#expectobjectcontainingobject):

    ```javascript
    // good
    expect(wrapper.props()).toEqual(expect.objectContaining({
      propA: 'valueA',
      propB: 'valueB',
    }));

    // better
    expect(wrapper.props()).toMatchObject({
      propA: 'valueA',
      propB: 'valueB',
    });
    ```

## The JavaScript/Vue Accord

The goal of this accord is to make sure we are all on the same page.

1. When writing Vue, you may not use jQuery in your application.
   1. If you need to grab data from the DOM, you may query the DOM 1 time while bootstrapping your
   application to grab data attributes using `dataset`. You can do this without jQuery.
   1. You may use a jQuery dependency in Vue.js following [this example from the docs](https://vuejs.org/v2/examples/select2.html).
   1. If an outside jQuery Event needs to be listen to inside the Vue application, you may use
   jQuery event listeners.
   1. We avoid adding new jQuery events when they are not required. Instead of adding new jQuery
   events take a look at [different methods to do the same task](https://vuejs.org/v2/api/#vm-emit).
1. You may query the `window` object one time, while bootstrapping your application for application
specific data (for example, `scrollTo` is ok to access anytime). Do this access during the
bootstrapping of your application.
1. You may have a temporary but immediate need to create technical debt by writing code that does
not follow our standards, to be refactored later. Maintainers need to be ok with the tech debt in
the first place. An issue should be created for that tech debt to evaluate it further and discuss.
In the coming months you should fix that tech debt, with its priority to be determined by maintainers.
1. When creating tech debt you must write the tests for that code before hand and those tests may
not be rewritten. For example, jQuery tests rewritten to Vue tests.
1. You may choose to use VueX as a centralized state management. If you choose not to use VueX, you
must use the *store pattern* which can be found in the
[Vue.js documentation](https://vuejs.org/v2/guide/state-management.html#Simple-State-Management-from-Scratch).
1. Once you have chosen a centralized state-management solution you must use it for your entire
application. Don't mix and match your state-management solutions.